In various areas of the country where bearing pressures of soil are low or unsuitable for structural purposes, it is common to employ one or more elongated load carrying elements placed in the soil beneath the structure. These elements are referred to in the industry as a pile or piling. These devices for transferring load between a structure and the underlying earth are typically of concrete, steel, or timber construction.
A standard pile depends on either skin friction, i.e., adhesion of soil to the surface of the pile, or end bearing to carry the load. In order to increase the skin friction, and hence the bearing load capability of piles, the geometry of piles has been altered in various ways to improve the load bearing resistance of the pile. For example, U.S. Pat. No. 2,151,847 utilizes the concept of a tapered wedge pile with reinforcing ribs which may add additional resistance.
In some applications, it would be desirable to provide a pile with a high resistance to uplifting, or vertical loading, as well as with a high bearing load capability, such as, for example, in anchoring electrical transmission towers. In various regions, especially where there is only shallow soil above the bedrock, the resistance to vertical loading is particularly critical because of the limited depth on which the pile can be driven. Piles depending only on skin friction to resist vertical loading are particularly unsuitable when changing conditions result in expansion and contraction of the soil surrounding the pile.
One approach has been to provide the pile with a flange, generally in the form of a screw thread, to increase its resistance to vertical loading. Numerous configurations of such flanges have been designed for piles in order to maximize their vertical loading capabilities. Examples of such piles may be found in U.S. Pat. No. 3,797,257; U.S. Pat. No. 101,379; U.S. Pat. No. 996,688; U.S. Pat. No. 4,239,419; Japanese Pat. No. 57,130626; Japanese Pat. No. 57,96128; USSR Pat. No. 983,193; USSR Pat. No. 870,586; USSR Pat. No. 808,594; and USSR Pat. No. 863,767. A major disadvantage of such piles is that, because of their high pitch and/or thread depth, they must be rotated or screwed into the soil, and cannot be driven by conventional pile driving equipment. In addition, these piles are generally heavily reinforced to minimize torsional failure during their screwed insertion into the soil, increasing their cost of manufacture. Moreover, such screwed piles are usually quite heavy and awkward to transport and to erect for driving into the soil.
On the other hand, drivable piles provided with a helical screw flange, such as, for example, that described in U.S. Pat. No. 226,664, have had limited practical utility. The driving of such a pile with conventional pile drivers, except with small piles used to anchor only relatively small and lightweight structures, creates an unequal stress on one side of the pile, with the result that the pile bends or fractures upon driving.
There exists, therefore, a need for a pile drivable with conventional pile drivers which is easily and inexpensively manufactured, transported and installed, and which has a high resistance to upward vertical loading.